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Whyte notation

From Academic Kids

The Whyte notation for classifying steam locomotives by wheel arrangement was devised by Frederick Methvan Whyte and came into use in the early 20th century. Whyte's system counts the number of leading wheels, then the number of driving wheels, and finally the number of trailing wheels, this being the common pattern of the conventional steam locomotive.

Thus, a locomotive with two leading axles (and thus four wheels) in front, then three driving axles (six wheels) and followed by one trailing axle (two wheels) is classified as a 4-6-2.

It's important to stress that wheels, not axles, are what is counted in this system. Other classification schemes in use elsewhere (like the UIC classification) count axles.

The system had to be extended with the advent of articulated locomotives. The scheme generally adopted is that locomotives such as Garratts, where there are, in effect, two separate locomotives joined by a common boiler, are classified by using a plus sign in between the arrangements of each engine. Thus, a 'double Pacific' type Garratt is a 4-6-2+2-6-4.

Simpler articulated types such as Mallets, where there are no unpowered axles in between powered axles, are just written by adding extra numbers in the middle; each number represents a grouping of wheels. Thus a Big Boy is written under this modified Whyte notation as a 4-8-8-4; there are two leading axles, one group of four driving axles, another group of four driving axles, and then two trailing axles.

In addition the suffix 'T' is often used to indicate a tank locomotive (otherwise, a tender locomotive is assumed). In British practice, this is sometimes extended to indicate what type of tank locomotive. When this is done, a plain 'T' means side tank, 'ST' means saddle tank, PT means pannier tank and WT stands for well tank. Where a 'T' suffix is followed by '+T', this indicates a tank locomotive that has a tender for additional coal or water capacity.

The limitations of the Whyte system for classifying locomotives that did not fit the standard steam locomotive pattern led to the design of other forms of classification. Most commonly used in Europe is the UIC classification scheme, based on German practice, which can more completely define the exact layout of a locomotive.

In American (and to a lesser extent British) practice, most wheel arrangements in common use were given names.

Here is a list of the most common wheel arrangements: in the illustration, which should be read left to right, with the front of the locomotive to the left, small o is a carrying axle, and a big O is a driving axle.

ArrangementWhyte ClassificationName
Non-Articulated Locomotives
Oo0-2-2  
oO2-2-0Planet
oOo2-2-2Single, Jenny Lind
oOoo2-2-4  
ooO4-2-0Jervis
ooOo4-2-2Bicycle
ooOoo4-2-4  
oooO6-2-0Crampton
OO0-4-0Four-Coupled
OOo0-4-2  
OOoo0-4-4  
oOO2-4-0Porter
oOOo2-4-2Columbia
oOOoo2-4-4  
ooOO4-4-0American, Eight-wheeler
ooOOo4-4-2Atlantic
ooOOoo4-4-4Reading, Jubilee (Canada)
OOO0-6-0Six-Coupled
OOOo0-6-2  
OOOoo0-6-4  
oOOO2-6-0Mogul
oOOOo2-6-2Prairie
oOOOoo2-6-4Adriatic
ooOOO4-6-0Ten-Wheeler (not Britain)
ooOOOo4-6-2Pacific
ooOOOoo4-6-4Hudson, Baltic
OOOO0-8-0Eight-Coupled
OOOOo0-8-2  
oOOOO2-8-0Consolidation
oOOOOo2-8-2Mikado, Mike, MacArthur
oOOOOoo2-8-4Berkshire, Kanawha
ooOOOO4-8-0Mastodon, Twelve-Wheeler
ooOOOOo4-8-2Mountain, Mohawk
ooOOOOoo4-8-4Northern, Niagara, Confederation, Dixie, Greenbrier, Pocono, Potomac
oooOOOOooo6-8-6(Used only by the Pennsylvania Railroad's steam turbine locomotive)
OOOOO0-10-0Ten-Coupled, (rarely) Decapod
OOOOOo0-10-2Union
oOOOOO2-10-0Decapod
oOOOOOo2-10-2Santa Fe
oOOOOOoo2-10-4Texas, Selkirk (Canada)
ooOOOOO4-10-0Mastodon, Gobernador
ooOOOOOo4-10-2Southern Pacific, Overland
OOOOOO0-12-0Twelve-Coupled
oOOOOOO2-12-0Centipede
oOOOOOOo2-12-2  
oOOOOOOoo2-12-4  
ooOOOOOOo4-12-2Union Pacific
ooOOOOOOOoo4-14-4 AA20
Duplex Locomotives
ooOO OOoo4-4-4-4Duplex
oooOO OOooo6-4-4-6Pennsylvania
ooOO OOOoo4-4-6-4(PRR Q2)
ooOOO OOoo4-6-4-4(PRR Q1)
Mallet and Simple Articulated Locomotives
OO-OO0-4-4-0  
o-OO-OO-o2-4-4-2  
OOO-OOO0-6-6-0Erie
oOOO-OOO2-6-6-0Denver & Salt Lake
oOOO-OOOo2-6-6-2  
oOOO-OOOoo2-6-6-4Norfolk & Western
oOOO-OOOooo2-6-6-6Allegheny
ooOOO-OOOoo4-6-6-4Challenger
oOOO-OOOO2-6-8-0Southern, Great Northern
OOOO-OOOO0-8-8-0Angus
oOOOO-OOOO2-8-8-0Bull Moose
oOOOO-OOOOo2-8-8-2Chesapeake
oOOOO-OOOOoo2-8-8-4Yellowstone (and, running in reverse, SP Cab Forward)
ooOOOO-OOOOo4-8-8-2(Southern Pacific cab forward)
ooOOOO-OOOOoo4-8-8-4Big Boy
oOOOOO-OOOOOo2-10-10-2(Santa Fe and Virginian RR's)
oOOOO-OOOO-OOOO-o2-8-8-8-2Triplex (Erie RR)
oOOOO-OOOO-OOOO-oo2-8-8-8-4Triplex

Garratts are almost always two identical locomotive frames back-to-back, and are thus called Double Pacifics, Double Northerns etc.

See also

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